Project Abstract Sleep apnea-hypopnea syndrome (SAS) is common and has a higher prevalence in the elderly relative to middle-aged individuals. This proposal is concerned with investigating the age-specific differences in susceptibility to breathing instability and upper airway susceptibility to collapse during sleep. The following specific aims will be investigated. Specific Aim 1 is to determine the age-specific effect on periodic breathing susceptibility during sleep. To this end, we will use non-invasive positive pressure ventilation (NIPPV) in the pressure-support mode to induce periodic breathing (as an index of breathing instability). Specific Aim 2 is to determine the age-specific effect on upper airway mechanics during sleep. We will test the hypothesis that pharyngeal retro- palatal compliance and the passive critical closing pressure are higher in older adults as compared to young adults. Therefore, we will measure the critical closing pressure, under baseline conditions and under hypotonic (passive) conditions to establish differences in collapsibility with, and without, active neuromuscular control. Using naso- pharyngoscopy, we will also measure pharyngeal compliance during sleep. Specific Aim 3 is to determine the age-specific effects of reduced ventilatory motor output and upper airway patency during NREM sleep. We hypothesize that expiratory pharyngeal narrowing is more pronounced in older adults as compared to young adults. We will investigate whether the pharyngeal cross-sectional area under conditions of induced hypocapnic hypopnea, in the aftermath of nasal mechanical hyperventilation, is smaller in older adults. The proposed studies address a significant clinical problem and will further our understanding of the pathogenesis of sleep apnea